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NCERT Solutions for Class 9th Science Chapter 6 .6 Newton's Third Law of Motion — Activity 6.5: Let us explore

Book page 1076 Updated on2026-09-08

Q1.
Now, using both your hands, push the table away from you, i.e., apply a force on the table in the forward direction as shown in Fig. 6.23a. What happens to you? Does the chair you are sitting upon move in the opposite direction?
Answer

You (with the chair) move backwards — yes, the chair moves in the direction opposite to your push.

You push the table forwards with force F
The table pushes you backwards with the same force F
Your feet are off the floor, so no friction holds you
Net force on you + chair = F backwards → you accelerate backwards
Why it happens: a force never acts alone — at least two objects must interact. When your hands press on the table, the table presses back on your hands with an equal and opposite force. The heavy table hardly budges because of its large mass and the friction under it, but you are on wheels, so the backward force on you is essentially unopposed and you roll away.
Tip: lifting your legs off the floor matters. With your feet down, friction from the floor would balance the table's push and you would not move.
Q2.
Now, try to pull the table towards you, i.e., apply a force on the table in the direction opposite to that in step 2 (Fig. 6.23b). In which direction does your chair move now?
Answer

The chair moves forwards, towards the table.

You pull the table towards you (backwards, from the table's point of view)
The table pulls you towards it with an equal force
Net force on you + chair points towards the table → you roll forwards
Why it happens: the direction of the reaction force always flips with the direction of the action force, because the pair is equal and opposite. Push the table away and it pushes you away; pull it towards you and it pulls you towards it. Both times the force on you is opposite to the force you apply on the table.
Q3.
What conclusion can you draw from this activity?
Answer

Whenever you exert a force on the table, the table exerts an equal force on you in the opposite direction — at the same instant. This is Newton's third law of motion.

Whenever one object exerts a force on a second object,
the second object simultaneously exerts an equal and opposite force on the first object
The key point students miss: the two forces of the pair act on two different objects — one on the table, one on you. That is why they do not cancel each other and why you actually move. Two equal and opposite forces balance only when they act on the same object.
Try This: the same thing explains why you can start a bicycle moving without pedalling by pushing the ground backwards with your feet, and why you walk at all — your foot pushes the ground back, and the ground (through friction) pushes you forward.
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